Supercritical Biodiesel Production from Raw Soybean Oil

This paper describes the supercritical methanol transesterification of raw soybean oil for biodiesel production in a batch stirred tank reactor without catalyst and looks at the main variables affecting the yield of biodiesel: the methanol-to-oil molar ratio and reaction temperature. Experiments were carried out for 20 min in the temperature range 250–350°C at methanol-to-oil molar ratios of between 20:1 and 53:1. The reactions were monitored by gel permeation chromatography (GPC) to determine the content of monoglycerides, diglycerides and triglycerides, and by gas chromatography (GC/FID) to analyse the individual methyl esters and to calculate the yield of biodiesel. The reaction conditions that led to good yields of biodiesel (72%) were a temperature of 350°C and a methanol-to-oil molar ratio of 45. There was no undesirable thermal decomposition of the fatty acid chains in these conditions. Due to the risk of thermal cracking above 350°C, reaction times need to be lengthened to complete the transesterification of monoglycerides and diglycerides remaining in the medium and so provide greater yields of biodiesel.